In a typical metallurgical process for the recovery of copper, arsenic present in the copper ore often remains in the refined products and/or in the process wastewater. Such arsenic-containing residues present significant cost and operational challenges to the copper recovery process as the residues must be treated or disposed of as hazardous materials to avoid negatively impacting the environment. Therefore, plants and/or refineries producing copper concentrates and products containing more than 5% arsenic must generally pre-treat these concentrates to remove arsenic.
A conventional process for pre-treating an arsenic-containing copper concentrate or product comprises heating the concentrate or product under reducing conditions to volatilize the arsenic as a sulfide. The arsenic sulfide is then combusted with air to produce arsenic trioxide and sulphur dioxide as follows:
The resulting calcinated material typically has an arsenic content ranging within commercial standards.
In an alternative process, arsenic-containing copper concentrates are smelted and reduced in converters or flash-type ovens to volatilize arsenic and produce an arsenic-containing off-gas. The arsenic-containing off-gas is further treated in a gas washing plant to recover the arsenic. The arsenic-containing off-gas is contacted with an acid to generate an effluent in the form of an acid solution. Conventionally, the acidic gas wash effluent is then neutralized with lime, filtered and dried to produce a calcium arsenate sludge. However, the handling and disposal of such arsenic-containing sludge are still governed by stringent environmental regulations. Therefore, a need exists for a more cost effective process wherein arsenic can be recovered from an acidic aqueous solution and reused as a salable commercial product while minimizing the risk of direct and indirect environmental pollution.
Many prior art processes are known for treating and/or recovering arsenic from wastewater and other sources. For example, in U.S. Pat. No. 4,138,231, Hedenas et al. describe a procedure for wet-cleaning gases containing sulfur dioxide, arsenic and halogens produced in the pyrometallurgy of sulfidic materials. The gases are washed with diluted sulfuric acid in a closed loop and arsenic is recovered as an arsenic trioxide product by precipitating arsenic trioxide with sulfides or by alkalinizing the solution. However, the process requires that the washing liquid be pregnated with at least 50 g/L of solid arsenic trioxide in order to promote arsenic recovery from the roaster gas.
Kohno et al., in U.S. Pat. No. 4,588,564, describe a process for removing arsenic from the exhaust gas of a sulfide ore smelter. The process discloses scrubbing the exhaust gas with water at a predetermined concentration and temperature to provide an effluent solution having an arsenic concentration of at least 80% of its saturation concentration. The solution is then filtered and arsenic trioxide is crystallized under vacuum or by cooling at a pH less than 2. Finally, the arsenic trioxide crystals are contacted with hydrochloric acid to produce an arsenic trioxide product.
Tomita, et al., in J. Metal, Vol. 35, No. 12 (1983), describe a process for recovering arsenic trioxide from copper cement. The process requires the steps of leaching copper cement to form CuSO4 in acid solution; dissolving arsenious sulfide in the CuSO4 solution; oxidizing As(III) to As(V); reducing As(V) to an As2O3 crystal with SO2 gas; and drying the arsenic trioxide crystals.
Madsen et al., in U.S. Pat. No. 4,401,632, describe treating gases from smelter flue dusts by forming an aqueous slurry of the dust, treating the slurry with sulfur dioxide gas to solubilize arsenic and precipitating arsenic as an arsenic trioxide product by means of sulfuric acid.
Yen, in U.S. Pat. No. 5,338,460, describes the removal of dissolved heavy metals, including arsenic, from aqueous solutions, particularly industrial and mining waters. The procedure discloses reacting the dissolved heavy metal with an inorganic sulfide or hydrosulfide at high temperature between 0° and 100° C. and controlling the pH between 2 and 3.5.
Allgulin, in U.S. Pat. No. 4,566,975, describes treating aqueous solutions containing impure heavy metals such as arsenic, phosphorus and mercury. The reference describes precipitating impure heavy metals from solution in two stages, using sodium hydroxide and ferric sulfate as precipitating agents. Arsenic impurities are then recovered as a sludge of iron-arsenic oxide, which can be stabilized and disposed.
Reynolds et al., in U.S. Pat. No. 4,244,977, describe the recovery of arsenic from ferric arsenate produced in the processing of materials containing high arsenic values, such as smelting powders. The procedure requires treating ferric arsenate compounds with sodium hydroxide, extracting pentavalent arsenic in a sodium arsenate solution and recovering an arsenic product.
Chen, et al., in Env. Sci. of China, 19(4):310–12 (1999) describe the removal of arsenic sulphuret from acidic metallurgical wastewater. The sulphuret is treated with acid to recover arsenic trioxide with a purity of 99.4%, along with a sulphur by-product.
Therefore a need continues to exist for recovering arsenic from aqueous acid solutions wherein a commercial arsenic trioxide product can be obtained, together with other by-products, which can be used directly in other processes and wherein the process does not produce any liquid or solid residues requiring special handling or disposal.